An enterprise customer may implement a Private Numbering Plan (PNP) to enable its employees in various geographical locations to communicate with each other by dialing extension numbers rather than standard telephone numbers such as E.164 standard numbers. That is, PNP enables each phone in each enterprise location to reach any other phone in any other of the enterprise's locations by dialing an extension number. For example, employees of an enterprise customer may be able to reach each other by dialing a four digit extension number instead a 10 digit number.
It should be noted that E.164 is an ITU-T recommendation which defines the international public telecommunication numbering plan, i.e., for formatting telephone numbers such that they may be signaled across one or more networks. The E.164 format includes a country code and subsequent digits, but not the international prefix. However, an extension number in accordance with a PNP is not associated with an E.164 standard number and no Telephone Number Mapping (ENUM) query may be issued to resolve such number. ENUM is a standard protocol defined by the Internet Engineering Task Force (IETF) for translating phone numbers that are in E.164 format to Internet domain names and vice versa such that a Domain Name Server (DNS) may resolve the addresses for E.164 numbers the same way it resolves traditional website domains. For example, ENUM may be used to transform a phone, a fax or a pager number into a URI (Uniform Resource Identifier).
In addition, in VoIP networks, a user may be assigned a username, e.g., johndoe@companyC.com, without being assigned a telephone number. Unfortunately, a caller from outside of the enterprise customer's network with knowledge of a username or an extension number is unable to reach the number directly. For example, a caller may have to dial a main number for the company (switchboard) and ask an operator to connect the call to the intended extension number.